Project Summary The way a cortical neuron processes sensory information is determined by its functional synaptic input circuit. This circuit contains three elements: 1) input connectivity from excitatory and inhibitory presynaptic neurons; 2) the strength and dynamic properties of each input; 3) processing properties of each presynaptic neuron. Although separate deciphering of each of these elements helps to extract how excitatory and inhibitory inputs interact to generate of output response properties of the neuron, an integral study addressing all these components in an intact system is necessary but remains to be a tremendous challenge. In vivo whole-cell voltage-clamp recording provides a unique and valuable approach for us to directly isolate and reveal the summed functional excitatory and inhibitory synaptic inputs under specific sensory stimuli. As the spatiotemporal properties of excitation and inhibition are determined by the above circuit elements, it provides a means to bridging the gap between connectivity with function. In this project, we will continue to harness the strength of this approach to understand various visual processing functions, and extend its application to awake mouse primary visual cortex (V1). First, we will determine the tuning relationship between inhibition and excitation underling orientation selectivity and spatial receptive fields in excitatory neurons in different cortical layers. Using neuron modeling and dynamic clamp recording, we will examine the diverse roles of inhibition in shaping functional selectivity. Next, based on our recent discovery of an interesting correlation between the direction tuning of excitatory responses under moving stimuli and the spatial asymmetry of excitatory input strengths evoked by stationary stimuli, we will examine how direction selectivity can be generated de novo in the cortex by testing a novel hypothesis that the spatial asymmetry can be converted into differential temporal summation under stimuli of opposite directions. By optogenetic silencing of cortical excitatory neuron spiking, the origin of this spatial asymmetry will also be examined. Finally, through optogenetics assisted cell identification, we will apply the whole-cell voltage-clamp recording to PV inhibitory neurons, and determine the mechanisms for their generally weak selectivity. Together, the proposed experiments will generate important new insights into how functional cortical synaptic circuits are organized and how cortical processing and sensory perception may go awry under neurological disease conditions which result in disrupted excitation-inhibition balance.